(DOCSP-30752): Migration patterns page (#20)

* (DOCSP-30752): Migration patterns page

* use abbr for CDC

Author: jeff-allen-mongo

source/index.txt

@@ -146,6 +146,7 @@ Explore libraries and tools for MongoDB.
    :hidden:
 
    /introduction
+   /migration-scenarios
    /installation
    /projects/projects
    /mapping-rules/mapping-rules

source/migration-scenarios.txt

@@ -0,0 +1,77 @@
+.. _rm-migration-scenarios:
+
+=================================
+Migration Scenarios and Use Cases
+=================================
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 1
+   :class: singlecol
+
+You can use Relational Migrator to migrate one legacy application at a
+time to MongoDB. Depending on your application's needs, you can migrate
+either with or without downtime.
+
+Migrate during Scheduled Downtime
+---------------------------------
+
+If your application does not require constant uptime, such as in the
+case of an internal application, you can have the migration occur during
+scheduled downtime. Migrating during downtime is the easiest way to
+configure and complete the migration process.
+
+When you start the migration process, Relational Migrator takes a
+snapshot of the data on the legacy application and migrates the data
+from that point in time. During the migration process, your legacy
+application can remain online to support incoming reads, but cannot
+support incoming writes. The length of the migration process depends on
+several factors, such as the amount of data being migrated and your
+connection speed.
+
+After the migration completes, take your legacy application offline and
+launch your new application.
+
+Migrate without Downtime
+------------------------
+
+If your application must remain online to support reads and writes
+during the migration process, you can use Change Data Capture (CDC) to
+migrate data without downtime. This process is called a **continuous
+sync migration**.
+
+When you start a continuous sync migration, Relational Migrator takes a
+snapshot of the data on the legacy application and tracks database
+updates in near real time. During the migration process, your source and
+destination databases remain in sync.
+
+After the continuous sync migration completes, you can transfer users to
+the new application and shut down the legacy application.
+
+To learn more about sync jobs, see :ref:`rm-sync-jobs`.
+
+Currently Unsupported Migration Scenarios
+-----------------------------------------
+
+Relational Migrator currently does not support the following migration
+scenarios:
+
+- Migrating multiple applications through a phased migration without
+  downtime. In this scenario, multiple legacy applications are
+  decommissioned and migrated to the new database while the original
+  database still supports writes. The migration uses :abbr:`CDC (Change
+  Data Capture)` to keep the legacy application in sync with the
+  destination database.
+
+- Migrating an operational data store. Depending on your workload, this
+  scenario could involve :abbr:`CDC (Change Data Capture)` running
+  indefinitely without completing.
+
+The preceding scenarios require long-running :abbr:`CDC (Change Data
+Capture)`, or in some cases involve :abbr:`CDC (Change Data Capture)`
+running indefinitely. Therefore, these scenarios are not recommended
+with Relational Migrator.
+
+Relational Migrator will support more migration scenarios and use cases
+in future releases.